Please don't leap to the idea that she needs to swallow a pill, even a vitamin. There is no magic pill for being nine. It is totally typical for kids this age to have things "go in one ear and out the other." It does not mean she has any memory issues; she has typical nine-year-old attention issues (and no, that does not mean she needs some diagnosis of some condition -- unless being nine is a diagnosable condition).
What exactly is the issue? For instance, do you tell her to do what seem like simple tasks like, "Please unpack your school bag and put your books on the desk, now" and she does not do it? That sort of thing? Then you try this:
Ensure she is facing you and looking straight in your face -- do not tell her things or issue orders as she is walking away or as you are in the kitchen and she's in another room.
After you tell her something, immediately have her repeat what you say back to you. "Honey, please let me know you heard me by telling me what I just said." Then after she says it, you say, "Thanks. That's right. Now please go do that and come see me when you are done."
Give her five minutes or whatever it takes and then check on her: "How is X going?" If she says, "Oh, I haven't done that yet," tell her nicely, "I did ask you to do it now. I'll set a timer." Then do just that.
It is not fun for you; it is tedious; but it can help. You also can try rewarding her for doing things promptly (kids her age respond well to rewards). But she needs you to work with her on her behavior and listening skills; vitamin supplements won't make a difference in her skills; only practice does that.
If she has issues remembering things for school, again, typical. Can she remember what she WANTS to remember and what's fun for her, such as an upcoming birthday party she wants to attend, when her favorite show is on, the details of her favorite novel's plot? Then she is mentally fine -- she is just being her age. Talk to her teacher about your child's listening skills at school and whether she needs help there too. But really, the in one ear and out the other problem is going to be with you for a long time.